The present application relates to a reflector body, in particular a spherical retro-reflective marker, which has a plurality of reflector body segments, a reflector body segment for producing such a reflector body, and a method of producing the reflector body segment and the reflector body.
Among others, retro-reflective bodies are presently used in the fields of healthcare, film business and in the computer industry for various applications such as performing a three-dimensional position detection. In this connection, retro-reflective bodies are used as applications for image-supported surgery, for monitoring a movement in radio surgery, for optimizing a motion sequence for elite athletes, for so-called feedback motion sequences in rehabilitation, for recording motion sequences in the field of the so-called “motion capturing” and for localizing people and tools for “virtual reality” images.
Furthermore, applications of retro-reflective materials in road traffic are known. WO 98/00737 A1 describes a street post having three strips with lenticular elements.
In contrast to a diffuse reflection from a surface, retro-reflection means that the reflected light rays are directly reflected back in the same direction, i.e. back to the light source, substantially parallel to the incident light rays. This is independent of the angle of incidence until a limiting angle is reached where the above-illustrated spheres cover each other.
In order to determine a position, the retro-reflective bodies are illuminated by a light source such as a conventional flash or an infrared flash ring, and reflect light in a very narrow range of angles with respect to the direction of incidence back to the light source. The illuminated scene can be filmed for example by a camera. Due to retro-reflection, the retro-reflective bodies have—similar to a light source—a much more intensive brightness in the recorded image than the non-reflective and darker environment. The retro-reflective bodies can therefore easily be isolated from surrounding objects and detected by software.
The retro-reflective bodies can be fixed at spatially fixed reference points and/or body parts and/or instruments or tools for example by a clamping or screwing device. When mounted on instruments or tools, the bodies follow the course of movement of the fixed bodies or of the instruments or tools.
By using film recordings, a motion sequence and/or a position of the tool with respect to the surroundings, in particular with respect to reference points, such as fixed reference points or reference points which are attached to body parts, can be recorded and calculated.
For example, when choosing a suitable line of sight, the position of an instrument with respect to a body part and/or a reference point can be determined unambiguously in three dimensions when at least three markers are arranged at the instrument in a fixed relationship.
Currently, both spheres and flat markers are used as retro-reflective bodies.
In general, flat markers are more accurate than marker spheres. This is due to the fact that retro-reflection films are used for producing spherical retro-reflective bodies. Known retro-reflective spherical bodies which are produced by using retro-reflection films show different reflection properties when illuminated from different directions, which is a disadvantage for determining a position.
One disadvantage of flat markers is, however, the strong limitation of the visible area when watching the scene from a lateral position. It is known to use a composite of four flat markers which are oriented under an angle to each other in order to combine the advantages of flat markers with the advantages of marker spheres. This requires, however, a much greater effort and results in significantly higher production costs.
Therefore, in most applications spherical retro-reflective bodies are used.
U.S. Pat. No. 3,964,820 describes a spherical retro-reflective element which retro-reflects a light ray when light is incident on its partially spherical surface portion. On the rest of the surface of this element, a plurality of concave and reflecting partially spherical small spheres is provided which reflect light rays which are incident on the partially spherical surface portion of the element and which are diffracted into the interior of the element.
U.S. Pat. No. 3,971,692 describes a retro-reflective material which is formed by a layer of transparent glass spheres on an adhesive coating formed on a carrier sheet wherein the exposed portions of the spheres are coated with a reflective material such as aluminum and the coated parts of the spheres are embedded in an adhesive coating of a component. Subsequently, the carrier film is removed together with the adhesive coating.
U.S. Pat. No. 4,265,938 describes retro-reflectors as used for road signs and license plates of vehicles. Here, the surface of a metallic substrate is retro-reflectively coated by applying a layer of an organic polymer and glass spheres and then by feeding the substrate through a roller where the glass spheres are covered with a crucible such that in the metallic surface of the spheres, depressions are generated.
FR 2 706 045 describes retro-reflective sheets and their production, the retro-reflective sheets especially being used for inspecting, measuring or observing a surface of an object. Here, a first reflective coating is applied to a metallic inner body of the sheet, and a second coating with glass spheres is applied to the surface of the first reflective coating.
EP 1 639 958 describes the production of a retro-reflective marker which is used in the medical field for surgery. A hollow sphere, preferably made of plastic, is coated with a retro-reflective layer which consists of a mixture of glass spheres, aluminum, gold or silver powder, by spraying or applying this mixture onto the sphere surface. The marker spheres can be attached to a support element.
WO 01/26574 describes a plastic sphere having a diameter of 10 mm to 15 mm to which a retro-reflective film is adhesively bonded.
DE 100 29 529 describes a reflector system for determining the position of instruments and devices, preferably for determining the position of a medical instrument or device in which the reflector consists of a transparent sphere which is either partially transparent or completely mirrored.
EP 1 640 750 describes a retro-reflective body and the production of such a body, which is especially used in the medical field for determining the position of surgical instruments. The body which has for example a spherical shape consists of a mixture of plastic and reflectively coated glass elements. The body is molded in an injection molding process. At the surface, the reflectively coated elements protrude from the plastic mass. The protruding coated elements and the reflective surface are separated by an etching process. A disadvantage of this process is that the coated glass elements are neither definitely oriented nor do they protrude uniformly from the plastic mass. Thus, the achievable retro-reflectivity as well as the homogeneous distribution of the retro-reflective glass elements is significantly lowered.
FIGS. 10a, 10b, 10c and 10d show various photographic images of a prior art spherical retro-reflective body where two hemispheres were covered with a film and then assembled. In the following, the assembling edge of the two halves running along a circular line will be referred to as the equator. The two points of the sphere which are intersected by a straight line which in turn runs perpendicularly to the equatorial plane through the center of the equatorial plane, are referred to as poles of the sphere. FIG. 10c shows the sphere from a viewing direction which is substantially perpendicular to one pole of the sphere. FIG. 10d shows the sphere from a viewing direction which is substantially perpendicular to a point in the equatorial region of the sphere. The two photographs were taken under identical conditions, i.e. at the same distance and with the same illumination of the sphere. The photographs clearly show that in the viewing direction as shown in FIG. 10c, the sphere reflects the light clearly better than in the viewing direction which the photograph in FIG. 10d is based on.
FIG. 10b shows a magnification of the sphere surface near one of the poles. FIG. 10a shows a magnification of a portion of the spherical surface near the equator. In these images, the exposure is adjusted to recognize the retro-reflective glass elements (micro-spheres) better. It is evident from these figures that the retro-reflective glass elements are closer to each other in the pole region than in the equatorial region which causes that in the viewing direction of FIG. 10c, the sphere reflects light better than in the viewing direction which lies in the equatorial plane.
In order to quantify the difference between the reflection properties, the average gray value of a surface area of the spheres can be determined. In order to avoid disturbing influences as for example a deterioration which is due to a viewing direction oblique with respect to the surface and which is caused by a shading effect, the gray values can be determined in a limited reference surface area R of equal size which includes a surface area substantially perpendicular to the viewing direction. The reference surface areas R chosen as examples and shown in FIGS. 10c and 10d are marked by dashed lines each. Given a spherical diameter of about 12 mm, the reference surface areas R may have a circular diameter of around 1.6 mm which approximately corresponds to a surface of the reference surface area of about 2 mm2.
The gray value determined in each case can be determined for example by a CCD camera where the determined gray value of a pixel linearly depends on the amount of incident light such that by comparing the determined gray values, the precise ratio of the reflection properties is evident. In the example shown in FIGS. 10c and 10d, the retro-reflective body showed a relative difference reflection with respect to the selected reference surface areas R of 70%. This means that in a viewing direction towards the spherical retro-reflective body which is shown in FIG. 10d, the spherical retro-reflective body reflects no more than 70% of the light compared to the viewing direction of FIG. 10c. 
It is apparent from this that an important criterion for the quality of markers is the CIL (value of back reflection) reached, which can also be expressed in cd/lux/m2 (candela per lux per square meter). Depending on the application, the CIL should be above 300 cd/lux/m2. Preferably, the CIL should constantly be achieved over the whole retro-reflective surface such that the markers show a consistent reflection behavior regardless of the viewing direction in space. In many applications such as medical applications for positioning surgical instruments and devices, a lateral viewing angle with respect to the marker is desired, the angle being as large a possible.
Ways for determining the quantitative retro-reflection of a surface to be evaluated are described in the CIE standard 054.2-2001. CIE is an abbreviation for the International Commission on Illumination (in French “Commission Internationale de l'Eclairage”). The CIE was founded at the beginning of the 20th century. The organization is recognized by ISO as an international standardization body. Furthermore, various systems are available on the market for a standardized measurement of retro-reflection where usually digital images of the surfaces to be evaluated are taken under specific lighting conditions in the systems. Then, using software an average brightness (for example a gray value) of a surface area of the surfaces to be evaluated is determined from the digital images. Finally, the reflection properties of the surface area are quantified based on the average brightness.
Systems available on the market are generally intended for determining the reflection of larger areas such as traffic signs. Therefore, those systems eventually are not suitable for measuring smaller bodies.
In the following with reference to FIG. 11, a measurement system will be described with which relevant reflection properties of small reflector bodies, especially of spherical retro-reflective markers can be determined. The measurement system shown in FIG. 11 has been further developed compared to the system which with the images of figure series 10 were taken. Using the measurement system of FIG. 11, those pictures were taken which are illustrated in figure series 12 and 13. The corresponding measurements were based on the CIE standard 054.2-2001.
The measurement system shown in FIG. 11 comprises a high resolution CCD camera 4 with a telecentric lens 3 having a coaxial illumination unit. In the measurement system, the measurement object 1 is fixed at a fixedly mounted socket 2. In case of evaluating spherical retro-reflective markers, the markers are fixed in the measurement system such that the center of the sphere lies on the optical axis of the telecentric lens 3. The spherical marker can be arranged rotatably around the center of the sphere. Then the measurement object 1 is illuminated by the illumination unit 3 and recorded by the CCD camera 4. The recording can be evaluated for example by a so-called machine vision software in real time or later by using image processing programs.
In the measurements carried out on the reflector bodies of figure series 12 and 13, a circular measurement point was defined as a reference surface area R using a diameter in each case. The reference surface area R has a diameter equal to about one-eighth of the diameter of the marker.
Figure series 12 represents photographs and excerpts from photographs with which using the described measurement system, a marker was measured which was available on the market during the priority interval of the present application.
FIG. 12a shows a photograph of the marker, where the recording direction was directed to an equatorial area of the marker. In FIG. 12a, the equator itself is shown as a relatively dark and thus poorly reflective line. This equator line is formed by the assembling area of the two hemispheres which has no retro-reflective surface due to production tolerances. In the illustrated photograph, the optical axis of the CCD camera 4 with the telecentric lens 3—the axis directed to the spherical center of the marker—intersects the spherical surface in the equatorial area slightly off the equator, wherein the reference surface area was selected adjacent to the equator line such that the equator line does not intersect the reference surface area in order to avoid a wrong measurement result due to the non-reflective areas of the equator line.
In figure series 12, the reference surface area is shown as a circular area which is shown with increased brightness compared to the rest of the sphere surface. This increased brightness does not correspond to real recordings, but has subsequently been integrated into the photographs for purposes of illustration, i.e. to graphically highlight the reference surface area. The measurements, however, were based on the unaltered recordings without said increased brightness.
As mentioned above, the diameter of the reference surface area corresponds to about one-eighth of the sphere diameter. This selection of the diameter reduces influences on the reflection properties due to surface curvature to an extent that they are negligible.
FIG. 12b shows a photograph of the marker, where the recording direction was directed to a pole region of the marker.
FIG. 12c shows an enlarged area of the reference surface area from FIG. 12a. FIG. 12d shows an enlarged area of the reference surface area of FIG. 12b. 
The brightness values of the reference surface areas from figure series 12 were determined. It was determined that the CIL in cd/lux/m2 of the reference surface area in the equatorial region was in some cases more than 30% below the CIL in cd/lux/m2 of the reference surface area measured in the pole region. Therefore, the relative difference reflection was less than 70%.
Figure series 13 shows an improved reflector body which was recorded under identical measurement conditions and which is described in detail below in the description of the figures.